Process for production of formaldehyde



May 22, 1945.

PROCESS FOR PRODUCTION OF FORMALDEHYDE Filed April 16. 1942 INVE NTORWALL ENE/Q .DERBY BY 6W AT ORNEY Patented May 22, 1945 PROCESS FOR PRODUCTION OF FORMALDEHYDE Wallene R. Derby, Oakwood, Ohio, assignor toMonsanto Chemical Company, a corporation o! Delaware Application April16, 1942, Serial No. 439,223

11 Claims.

This process relates to the production of formaldehyde by the oxidationof low molecular weight aliphatic hydrocarbons.

Previously known processes for producing formaldehydeby the oxidation ofhydrocarbons have employed as catalysts either nitric oxide, nitrogenperoxide or a great variety of solid catalysts such as the oxides of thevarious metals. While a great deal of work has been done on theseprocesses the yields of aldehyde per cubic foot of hydrocarbon have beenuniformly low and for this reason have not been commercially developedto any great extent except in those instances Where a plentiful supplyof suitable hydrocarbons was available.

In general it has been found that the use of solid catalysts isunsatisfactory for this reaction. I prefer to employ the linown gaseouscatalysts such as those derived from nitric acid, e. g., NO, N203 andNO2. For the purpose of supplying these catalysts I may utilize nitricacid or substances capable of supplying oxides of nitrogen to thereacting gases. In addition to nitric acid, other usable substances areethyl nitrite, isopropyl nitrite or other alkyl nitrites.

I have also found, that in order to effectively oxidize hydrocarbons toformaldehyde, the oxidation reactions must be carried out in a chamberor tubeiwhich has been suitably treated so as to prevent objectionablecatalysis of the hydrocarbon oxidation reaction and also the furtheroxidation of the formaldehyde produced thereby. By objectionablecatalysis of the hydrocarbon oxidation reaction I means the undesiredconversion o'f the hydrocarbons to carbon dioxide and water. that iscomplete oxidation, of the hydrocarbons. A suitable material for theconstruction of the oxidation chamber is difcult to'iind due to thecomplexity of the oxidation reaction.

I have now found that excellent yields of formaldehyde are obtained if Iemploy a reaction chamber constructed of a nickel-molybdenumchromiumalloy, in which the proportions may be within the following limits:

Fe, W, Si, Mn Balance to make 100 A specific alloy which has been foundto be suitable for this purpose has the following composition:

Percent Nickel 55 Molybdenum 1'7 Chromium l5 Tungsten 4 Fe, Si, MnBalance the major portion of the balance being iron.

In comparable tests, in which tubes constructed of steel, stainlesssteel. nickel and the Ni, Mo, Cr alloy were employed as the reactionchamber, and when oxidizing propane-air mixtures, the folowing yields ofHCHO per cubic foot of hydrocarbon gas charged were obtained:

Steel 3 Grams per cu. it. of propane Stainless steel 5-6 Grams per cu.ft. of propane Nickel 3 Grams per cu. ft. of propane Ni, Mo, Cr alloy-12.2 Grams per cu. ft. of propane While my discovery may be appliedgenerally Where hydrocarbon oxidation reactions are carried out, it isof particular value where such oxidation reactions are being carried outunder substantially adiabatic conditions. An adiabatic reactor and theapplication thereof for the production of formaldehyde is described andclaimed in a copending application of C. A. Hochwalt, et al., Serial No.433,648, illed March 6, 1942. My invention may be employed therein, byconstructing theoxidation chamber, whether tubular or other- `wise of mypreferred alloy composition, as herein disclosed.

The reaction chamber within which the principal oxidation reaction iscarried out, should be heated, which is done by the external applicationof heat at a uniform temperature level. The attainment of a uniformtemperature of the reaction tube precludes the use of direct heat byfuels as such heat has ordinarily been applied. In place of such directapplication of heat, it has been found that the temperature may bemaintained more uniformly by the use of a liquid heating mediumsurrounding the chamber or tubes. By this means a uniform temperature ofthe tube or chamber is attained Without the use of an excessivetemperature gradient through the tube walls. The liquid heating mediumsurrounding the reaction tubes moreover provides not only a largereservoir of heat at constant temperature but also provides meanswhereby heat may flow freely through the walls thereof with a greatlydecreased lm temperature of the tube walls. The provision of a metallicmaterial of construction for the reaction chamber or tubes greatlyfacilitates the transfer of heat, which factor resultsin a considerablyincreased yield of aldehyde.

In general, various aliphatic hydrocarbons may be used in my process,including methane, natural gas, ethane, ethylene, propane, propylene,butane, etc. Such hydrocarbons are normally gaseous at ordinarytemperatures and pressures, however other volatile hydrocarbons, such asthe natural gasoline hydrocarbons may also be employed.

In carrying out the reaction for the production of formaldehyde byoxidation of low molecular weight hydrocarbons I mix together thegaseous hydrocarbon and air in the proportions of approximately byvolume of a hydrocarbon such as propane, together with about 85% byvolume of air. The proportions of air and hydrocarbon may be variedsomewhat, in any event it is desirable to keep the proportion ofhydrocarbon above the upper explosive limit. Other hydrocarbons mayofcourse be used in my process.

Natural gas, having for example the following compositions in percent byvolume:

Per cent Methane 83.5 Fithane 9,5 Propane y 4.4 N2, H2 and CO 2.6

is a cheap and readily available hydrocarbon for the present purpose.When employing this gas. a mixture thereof with air in the proportionsto 35% natural gas to 75% to 65% of air preferably say gas to 70% air ismade up and constitute the charge gas of the present process. Thecomposition of this gas-air mixture is above the upper explosive limit.

Such a hydrocarbon-air mixture is passed into a preheater which servesto preheat the gas mixture to within the range of 250 C.,to 350 C. with310 C.`being about the preferredrange when oxidizing natural gas.

To a separate preheater I supply the residual y gas mixture which hasalready passed through the reactor and from which the oxidizedhydrocarbons, i. e., formaldehyde have been largely removed byscrubbing. This gas is for convenience termed recycle gas and as statedabove is supplied to the preheater wherein it is raised to a temperaturewithin the range of 350 C. to 550 C., a temperature of about 480 C.being preferred when oxidizing natural gas. The preheatedhydrocarbon-air mixture and the preheated recycle gas are mixed togetherin the approximate ratio of 100 volumes of the hydrocarbon-air mixturetogether with from 200 to 300 volumes of preheated recycle gas mixture.A preferred ratio is 100 volumes of hydrocarbon-air to 260 volumes ofrecycle gas. The mixing of these two gases is conveniently done in sucha way as to conserve the heat in the gases and to produce a gas mixtureat a temperature in the neighborhood of 430 C. To this gas mixture isnow added a small amount of aqueous nitric acid vapor (also superheatedto approximately 430 C.) in such a proportion that the final mixturecontains about 1.2 volumes of 100% HNO.: vapor. Nitric acid may beemployed within the range of from 0.2

volume to 2.0 volumes of HNOa vapor per 200 volumes of total gasmixture.

After the nitric acid has been vaporized into the gas mixture theoxidation reaction starts. At this point the gases enter the reactorchamber. Some exothermic heat is liberated from the oxidizinghydrocarbon gases which heat serves to raise the temperature of thegases in the reactor. The gas temperature which is desirably finallyobtained is a temperature in the neighborhood of 750 C. A completeoxidation reaction where the reactants are oxidized completely to carbondioxide and water is undesirable and is prevented by control of twofactors; namely, (1) the time of sojourn in the reactor chamber, and (2)the maintenance of the Walls of the reactor chamber uniformly at atemperature not over about 575 C.

The first necessary condition mentioned above is readily obtained bycontrolling the velocity of gases passing through the reactor chamber.The second condition is obtained by surrounding the walls of the reactorchamber with a fluid transfer medium such as molten salts or moltenlead.

Although the temperature of gases entering the reactor may be somewhatbelow the temperature of the reactor and the temperature of the gasesleaving the reactor are generally somewhat above the temperature of thereactor, the over-all effect of the reactor chamber is to conserve theheat of the reacting gases without adding material amounts of heat tothe entering gases and also without withdrawing material amounts of heatfrom the exit gases. 'I'he reactor thus conforms to a more or lessadiabatic chamber maintained at the preferred temperature as statedabove and providing an optimum time of sojourn of the gases without amaterial net transfer of heat from the gases to the reactor.

It has been found that the optimum time of sojourn of the gases in thereactor ranges from 0.1 second to 0.2 -second and may be as high as 0.4second.

Because of the complete submergence of the tubes in the molten liquidbath at a constant temperature, some transfer of heat desirably occursfrom end to end of the reactor tubes through the bath. As pointed outabove, the

gases at the exit end of the tube are at a higher I temperature than thebath temperature, and accordingly some heat ilow from the hotter gasthrough the tube walls into the liquid bath, which thereby becomesheated in the zone adjacent said hotter tubes. At the same time theincoming gases are at a somewhat lower temperature than the bath andaccordingly some heat is transferred from the bath through the tubewalls and to the gases. However, the bath being fluid will tend toequalize itself as to temperature either by natural convection, bymechanical agitation or both. I may therefore so adjust the temperaturelevel of the reactor (the bath temperature) that the oxidation of thehydrocarbons is carried out to the desired extent without a material nettransfer of heat between the gases undergoing oxidation and the bathsurrounding the chamber or tubes.

The attainment of the above conditions is not independent of theconditions of preheat of the gases. It is accordingly necessary topreheat the gases to a degree such that the oxidation reaction may becarried out under substantially adiabatic conditions. It has been foundthat such a degree of preheating may be carried out without loss ofyield, by preheating the hydrocarbonair mixture separately from thepreheating of the "recycle gas. Preferably the hydrocarbonair mixture isheated to a point short of incipient g oxidation. The permissibletemperature will vary Per cent CH4-- C2 and higher hydrocarbons NO Suchgas contains less oxygen than the fresh hydrocarbon-air mixture, andconsequently may be preheated to a somewhat higher temperature, as abovementioned. However, due to the reactive nature of the gases when mixedit is undesirable in the present process to rst mix the hydrocarbon-airportion with the recycle" portion and then preheat the resultingmixture.

The gases upon leaving the reactor at a temperature of about 750 C.enter a primary cooler which may take the form of a steam boiler inwhich the gases contact boiler tubes which are maintained at atemperature of about 200 C. and which in turn cool the gases to atemperature of about 300 C. Upon leaving the cooler or boiler the gasesare passed to a secondary cooler where they are contacted with thecondensate obtained by cooling the gases and consisting mainly of anaqueous solution of formaldehyde. The temperature herein is furtherdecreased to in the neighborhood of from 30 C. to 40 C.

Upon leaving the secondary cooler the gases are passed to a scrubberwhere they are additionally washed with a small amount of pure water inorder to remove additional formaldehyde and then from the scrubber theypass to a blower which forces the gases, except for dischargeor exitgases, back to the preheater mentioned above. The gases leaving theblower when oxidizing methane have a volume of approximately 350 volumesand since the above preferred mix,-

ing proportions call for approximately only 260 volumes of "recycle gasthe excess gas amounting to approximately 90 volumes is discharged tothe atmosphere.

My process may be understood by reference to the accompanyingdiagrammatic ow sheet comprising the single figure of the drawing. Thetemperatures and gas volumes thereon indicated are those preferred whenoxidizing methane, or-

obtained from the'system by means of pipe I 8,

conveyed to preheater I9 wherein the temperature is raised to in theneighborhood of 480 C.

The preheated natural gas-air Vmixture and the preheated recycle gasmixture are conducted by pipes to mixing device in such amount as toconsist of approximately 100 volumes of hydrocarbon-air mixture and 260volumes of recycle gas. The resulting mixture will therefore consist ofa total of 360 volumes of mixed gas at a temperature of approximately430 C., that is, at an intermediate temperature between that of theconstituent gas mixtures. The gases, after leaving mixing device 20,pass by means of pipe 2i to anothery mixing device 22, at. which pointnitric acid vapor is introduced by means of pipe 48 connecting with thesource of nitric acid i2. A vaporizer for vaporizing the nitric acid andpreheating the vapors to the temperature of the gas with which it is tobe mixed is indicated at The nitric acid containing gas mixturethereafter passes by means of pipe 28 into thenickel-molybdenum-chromium metal tubes forming part of reactor 2t.Reactor 24.

consists of the alloy metal tubes which are preferably immersed in aliquid heating medium such as molten salts or molten lead, thetemperature of the liquid surrounding the tubes being maintained atapproximately 575 C., and preferably between 550 C. and 650 C. The timeof sojourn maybe between 0.1 second and 0.2 second but should not extendto as much as 0.4 second. The time of sojourn of the gases in thereactor is controlled by proportioning the volume of gases passedthrough the tubes to the volume of the reactor tubes.

The gases leave the reactor tubes by means of pipe 20 and pass directlyinto a primary cooler 26 which may take the form of a steam boiler. Inprimary cooler 30, the gases are cooled down to in the neighborhood ofabout 300 C. and thereafter leave by pipe 2l, passing directly into asecondary cooler 28. In secondary cooler 20 the gases are cooled bycontact with cooled condensate circulated through the secondary cooler.The condensate is removed from a lower point of the secondary cooler bymeans of pump t@ and pumped into cooler 30 wherein the temperature islowered approximately to that of the available cooling watertemperature. The circulated condensate, together with the newcondensate, leaves cooler 30 and flows by pipe 3i back into thesecondary cooler 20. The gases leave secondary cooler 2B by means ofpipe 32 and enter scrubber 33 winch may conveniently take the form of aplatecolumn. A small quantity of water is supplied by means of pipe 34to scrubber 33 and is discharged from the scrubber by pipe t6. Theproduct consisting of the condensate is withdrawn from pipe 30 whichconnects pump 29 to cooler 30 by means of pipe 31, and is then combinedwith the scrubber liquid flowing in pipe 35 forming the product of theprocess in pipe 38. The product of the process consists of an aqueoussolution of formaldehyde and aldehydes and acids which may thereupon becollected in tank 39.

When oxidizing \a natural gas consisting largely of methane, asdescribed above, the aqueous condensate will have the followingapproximate composition:

Per cent Formaldehyde 16.1

Methanol c l 2.3 Water plus small amounts of formic and nitric acidsBalance The gases leaving scrubber 33 are drawn through pipe 40 intoblower Il and then enter pipe 42 which returns recycle gas to pipe Il.At the same time exit gas is withdrawn by means of pipe from the systemin such an amount as to maintain a constant pressure in the system.

Some latitude is possible in respect to the method and temperature ofpreheating the gases. Thus, for example, while the preheat temperatureoi' the natural gas-air mixture may be employed with a contact time ofless than about 0.4 second in the reactor it is possible to increase theamount of preheat to a higherflgure than that givenabove vby suitablydecreasing the contact time. Since the point of incipient oxidation isdependent upon both temperature and contact time I may, however,increase the temperature to a higher degree by decreasing the contacttime in the preheater without encountering serious oxidation therein.Similarly, lower temperatures' may, if desired, be employed.

The same general considerations govern the temperature for preheatingthe recycle" gas. Thus higher temperatures such as 500 C. or even 550 C.may be obtained by appropriately adjusting the contact time. Lowertemperatures may likewise be employed. Moreover the proportions ofrecycle" gas used may be changed from that given above.

The general consideration governing the choice of preheat of the gasesis to assure the substantially adiabatic condition of the reactor duringoperation. The adiabatic state may, of course, be achieved at varioustemperature levels, within the limits stated, hence it is not desired tolimit the operation of the reactor to any particular temperature level.By appropriate choice oi preheater temperature and by adiusting theother variables in known manner, it is possible to arrive at a reactiontemperaturer which temperature will be generally within the limitsranging from 725 C. to 775 C.,l at which the reactor will, from a heatstandpoint, be substantially adiabatic.

This condition is'independent of the pressure of the gases within thereactor, hence the present process may be operated at atmosphericpressure, as well as at superatmospheric pressures. Pressures generallymay be, say 25, 50 or even 100 lbs. or more per square inch pressure.However, at pressures of 200 lbs. and over the course of the oxidationreaction is influenced unfavorabiy.

As stated the reactor is preferably constructed of nickel, molybdenum,chromium alloys, and immersed in a liquid heat transfer bath. The tubesshould be of at least l/L inch internal diameter and not ,substantiallylarger than 1 inch internal diameter. Within this range of sizes theadiabatic condition is readily obtained and maintained for satisfactoryoperation.

What I claim is:

1. In the process for producing formaldehyde by the partial oxidation ofnormally gaseous aliphatic hydrocarbons in which a gas mixturecontaining said hydrocarbon and air is oxidized and formaldehydeproduced, the step of reacting said gases during said oxidation in areaction zone, the surfaces in contact with said oxidizing gases beingformed of a nickel-molybdenum-chromium alloy.

'2. In the process for producing formaldehyde by the partial oxidationof normally gaseous ali phatic hydrocarbons in which a gas mixturecontaining said hydrocarbon and air is oxidized and formaldehydeproduced, the step of reacting said sob . gases in a reaction zoneformed by a metallic alloy having a composition substantially asfollows:

Percent Nickel 40-60 Molybdenum 25-10 Chromium 25-10 the balance of saidalloy being composed of metal selected from the class consisting ofiron, tungsten, silicon and manganese.

by the partial oxidation oi' a normally gaseous aliphatic hydrocarbon,in which a. preheated gas mixture containing said hydrocarbon isoxidized and formaldehyde produced, the step of reacting said gasesafter preheating in a reaction zone defined by surfaces composed of analloy having the balance of said alloy being composed of metals chosenfrom the class consisting of iron, tungsten. .I

silicon and manganese.

5. In the process for producing formaldehyde y the partial oxidation ofnormally gaseous aliphatic hydrocarbons,'in which a heated gas mixturecontaining said hydrocarbons and oxygen is reacted while flowing througha heated tubular reactionv zone forming formaldehyde, the step of 5maintaining said reaction in contact with surfaces composed of an alloyhaving the approximate composition:

Per cent Nickel 40-60 Molybdenum 25-10 Chromium 25-10 the balance ofsaid alloy being substantially iron.

6. The process defined in claim 5 in which the y alloy has a compositionapproximately as follows:

the balance being substantially al1 iron.

7. In the process for producing formaldehyde' Y by the partial oxidationof normally ygaseous ali- 65 phatic hydrocarbons in whichal heated gasmixture containing said hydrocarbons and oxygen together with catalyticamounts of nitric axide is oxidized to formaldehyde while flowingthrough a heated tubular reaction zone, free of solid catalyticmaterial, the said tubular reaction zone being maintained at anoxidizing temperature by immersion in a liquid heatytransfer medium, thestep of maintaining said gases in said zone by confining the same bymeans of surfaces composed of an alloy composed essentially of nickel,

molybdenum and chromium.

8. The process defined in claim 7 in which the tubular reactor ismaintained within the temperature range of from 550 C. to 650 C.

9. The process defined in claim 7 in which the tubular reactor ismaintained within the temperature range of from 550 C. to 650 C. and theoxidizing gases are permitted to attain a temperature within the rangeof 725 C. to 775 C.

prior to cooling.

the composition:

' Per cent Nickel 40-60 Molybdenum 25-10 25 Chromium 25-10 10. In theprocess for producing formaldehydev by the partial oxidation of normallygaseous hydrocarbons. in which process a heated gaseous mixturecontaining said hydrocarbons and oxygen is caused to react, and in whichreaction, formaldehyde together with complete' oxidation products ofsaid hydrocarbons are formed, the improvement which comprises carryingout said oxidation reaction in contact with an alloy comwhereby theamount of said complete oxidation products is decreased and the amountof form- 5 the reaction is carried out in contact with an alloy

